1. Field of the Invention
The present invention relates to a detection apparatus for monitoring the charge condition of a suspension of finely divided solid charged particles in a fluid, such as water. The results of the monitoring can be used for the control of a water treatment process, for example flocculation, the monitoring being carried out on the water after such treatment to provide a feedback control over the dosing with flocculant. The invention also relates to a fluid treatment system using such a detection apparatus.
2. Description of the Prior Art
It is well known to clarify different forms of process water containing various concentrations of suspended particles by a process of flocculation, in which the suspended finely divided particles are made to agglomerate together to form relatively large particles which can then be removed from the fluid medium by any one or more of a number of physical separation processes such as filtration, sedimentation etc. The process of flocculation involves the addition of a suitable flocculating chemical, chosen according to the known nature of the particles concerned. These chemicals have the effect of neutralising the normally charged particles, whereby the agglomeration of the particles is no longer inhibited by the forces of mutual repulsion which previously kept the particles separated. The success of the flocculation process relies heavily upon accurate control over the amount of flocculating chemical added to the suspension; if too little chemical is added charge neutralisation is incomplete, whereas the addition of too much chemical may cause reversal of the charges on the particles, resulting in the same fine division of the particle in the treated fluid as in the original untreated fluid. Moreover, over-dosing may result in further contamination of the fluid.
The traditional method of ensuring correct dosing was to test a number of samples of the untreated fluid with different amounts of the flocculating agent and to then proceed with the full-scale flocculating process employing a concentration equal to that giving optimum flocculation in the samples. It will be appreciated that this process was laborious, time consuming, and subject to inaccuracies when applied to a fluid medium flow with time-varying particulate concentration.
The need for a reliable instrument for accurate determination of particulate concentrations gave rise to a device which employs a phenomenon known as the streaming current effect, this device being known as a streaming current detector, and referred to herein as SCD. This effect involves the immobilisation of a layer of the charged particles on a surface and the rapid movement of further suspended particles over this layer, a resulting signal generated across electrodes spaced apart along the surface in the direction of particle movement having a magnitude dependent upon a number of factors, including the particle concentration. A number of instruments using the streaming current effect have been developed over recent years, one example being illustrated in FIG. 1 of the accompanying drawings.
The basic elements of this known construction of SCD are an SCD cell 1 mounted in a fluid flow housing 2, the fluid flow housing 2 having a fluid inlet 3 and a fluid outlet 4, with a fluid flow passage extending between the inlet and outlet for conducting fluid flow through the housing. The SCD cell 1 comprises a cylindrical tubular casing 5 mounted with its lower end in the fluid opening 3, and with its opposite end projecting outside the housing 2. Mounted within the tubular casing 5 at its lower end is a piston-receiving member 6 in the form of a sleeve of insulating material having a central bore which receives the lower end of a reciprocatable piston 7 disposed coaxially within the tubular casing 5. The upper end of the piston 7 extends through a cap assembly 8 fixed to the outwardly projecting portion of the tubular casing 5. The piston-receiving member 6 carries a pair of annular electrodes 9 which are spaced apart along the axis of the piston, and there is an annular gap 10 between the lower end of the piston 7 received in the member 6, and the cylindrical surface of the bore in this member 6. The tubular casing 5 is formed with two diametrically opposed openings 11 through which fluid from the fluid-flow passage within the housing 2 can pass toward and from the bore of the member 6.
The piston is driven by a motor 12 coupled through an eccentric element 13 and a universal coupling 14 to the upper end of the piston. A sensor 15 detects the rotational position of the eccentric member 13 and thus the longitudinal position of the piston 7.
In use, the test fluid containing finely-divided particles is supplied to the inlet 3 and passes upwardly around the outside of a base cap 16 which closes off the lower end of the tubular casing 5, and generally follows the path of the chain-dashed arrow toward the outlet 4. As the motor 12 rotates, the lower end of the piston 7 reciprocates within the bore of the member 6. The upward travel of the piston draws fluid which has entered the casing 5 through the openings 11, through the annular gap 10 into the member 6. The downward stroke of the piston causes reverse flow of the fluid out of the member 6, thereby displacing an amount of fluid from within the tubular casing 5 back into the fluid flow path through the housing 2.
Each stroke of the piston causes relative movement between particles adhering to the inside wall of the member 6 and particles travelling axially in the annular gap, thus generating a signal across the electrodes 9, this signal being coupled to external measuring equipment by means of conductors 17.
The fluid under test thus passes up from the fluid inlet 3 through the annular gap between the upper portion of the cap 16 and the cylindrical wall of the inlet bore, and thence along a tortuous passage as indicated by the chain-dot lines to the outlet 4.
The above-described equipment has been used satisfactorily for process waters of moderate concentrations of suspended particles, the problem of accumulation of particles being alleviated by the attachment of an ultrasonic device 18 to the housing 5 to generate mechanical vibrations in the liquid.
It has been suggested that when dealing with fluids with above average particle size and/or concentration a partial filtering of a sample flow taken from the main fluid flow be performed and the filtrate be supplied to the SCD for monitoring. An indication of the concentration in the main flow is then obtained by processing the SCD output in accordance with the known filtering characteristics of the filter. However, this process is somewhat cumbersome and produces a certain time delay between the sampling from the main fluid flow and the generation of the detection output for that sample.
These problems arising from high concentration of particles are encountered in such applications as water purification, and the treatment of domestic sewerage, sludge, industrial fluids and industrial effluents.
The present invention seeks to alleviate, at least partly, this problem.
We have found that the constructional geometry within the housing has a significant effect upon the flow of large particles through the cell, and have developed an improved arrangement having constructional features leading to significant operational advantages over the arrangements known hitherto.